1. Field of the Invention
The present invention relates to a battery charger for an electric vehicle, which charges a main battery for feeding drive power to a motor.
2. Description of the Related Art
Electronic vehicles are generally classified into an ordinary electric vehicle which uses only a motor as the drive source for running and a hybrid vehicle which uses both the engine and motor as its drive sources. The hybrid vehicle fundamentally requires no external charging but needs the motor power to start the engine. When the main battery which feeds power to the motor is discharged, therefore, it is not possible to drive the hybrid vehicle as well as the ordinary electric vehicle which has the motor as its sole drive source. Therefore, even a hybrid vehicle should be equipped with some means which can charge the main battery in an emergency, in addition to the normal vehicle-mounted charging system.
One example of known means for charging the main battery in an emergency is a charger which charges the main battery of a vehicle where the charger is installed by using both a DC 12-V battery source installed in this vehicle and a DC 12-V battery source installed in another gasoline-engine vehicle or the like. Japanese Unexamined Patent Application Publication (KOKAI) No. 9-284913 discloses a technique of eliminating a charger from a vehicle, thus improving the usage of the vehicle and making the vehicle lighter. This is specifically accomplished by installing a charging motor, which is drivable on a domestic supply voltage or a commercially available supply voltage, in a vehicle and supplying the charging motor with power from outside the vehicle to thereby charge the battery with the generator output.
When a charger is not installed in a vehicle, however, it is not possible to cope with an emergency case where a domestic supply voltage or a commercially available supply voltage cannot be used as an external supply voltage. Even if a charger which uses a DC 12-V battery designed for usage in an emergency is installed in a vehicle, on the other hand, this charger is not used frequently. If this charger fails, therefore, it is likely that a user does not notice the failure until its use in an emergency.
The present inventor proposed a solution to this shortcoming in Japanese Patent Application No. 10-300082. The charger according to this prior art comprises a DC-AC inverter which converts a DC input voltage to an AC voltage and boosts the AC voltage to a predetermined voltage level and a voltage doubler section which boosts the AC voltage output from the DC-AC inverter or an AC voltage input without intervening the DC-AC inverter to a doubled voltage and rectifies this voltage. This single charger can therefore allow the main battery to be charged with both a DC voltage supply and an AC voltage supply.
As it is not often that the main battery is charged, the voltage doubler section is not used frequently. When the voltage doubler section has an ordinary structure comprising diodes and capacitors, the capacitors are deteriorated if the voltage doubler section is not used over a long period of time. While the prior art proposed by the present inventor can improve the frequency of usage of the DC-AC inverter, the prior art still has a room for improvement on such deterioration of capacitors.
The capacitors that constitute a voltage doubler section are normally electrolytic capacitors. It is well known that if electrolytic capacitors are not used for a long period of time, the impairment of the chemical conversion coating is apt to be quickened in a loadless state.
If the main battery is to be charged in an emergency with the voltage doubler section unused over a long period of time, therefore, a large leak current may flow through the electrolytic capacitors of the voltage doubler section so that the output voltage does not rise to a specified level.